Mitochondrial DNA (mtDNA) mutations cause a number of severe maternally transmitted diseases, and the accumulation of somatic mtDNA mutations is implicated in aging and common diseases of the elderly. These mtDNA mutations often coexist with normal mtDNA, a condition known as heteroplasmy. The ratio of mutated to wild-type mtDNA plays a crucial role in the pathogenesis of heteroplasmic disorders, but the mechanisms that influence this ratio are largely unknown. The long-term objective of our work is to define the cellular mechanisms that govern the frequency of deleterious mtDNA mutations in heteroplasmic somatic tissues. Recent work on the Parkinson's disease-related factors PINK1 and Parkin has revealed that these factors are components of a mitochondrial quality control system (MQCS) that can detect dysfunctional mitochondria, and, in collaboration with other cellular factors, promote their autophagic degradation. We hypothesize that the MQCS acts to reduce the frequency of deleterious heteroplasmic mtDNA mutations by detecting dysfunctional mitochondria that bear mutant DNA and targeting them for degradation. To test this hypothesis, we propose to use the model organism Drosophila melanogaster to pursue three aims. The first aim will examine the influence of genetic alterations of the MQCS on the phenotypes of a Drosophila strain with an increased mtDNA mutation frequency. The second aim will examine the influence of genetic perturbations of the MQCS on the mtDNA mutation frequency using a novel next-generation sequencing method. Finally, the third aim will use a recently developed in vivo assay of mitochondrial turnover to test whether mtDNA mutations promote mitochondrial turnover, and whether genetic perturbations of the MQCS influence the effects of mtDNA mutations on turnover. Our studies will contribute to an understanding of the molecular mechanisms that influence heteroplasmy, and this knowledge could ultimately lead to the development of treatments for diseases caused by mtDNA mutations.